Rigging in rigify for custom rig
You can absolutely customize your Rigify Meta Rig which would allow you to rig (almost) any character. The "secret" is in understand the Building Blocks or "rig types" that Rigify provides.

I made a tutorial series on exactly that. Here is the series page

https://cgdive.com/rigify

Or the youtube playlist

https://www.youtube.com/playlist?list=PLdcL5aF8ZcJv68SSdwxip33M7snakl6Dx

I hope this is a reasonable "self-promotion". There is very little info on this topic. I learned the basics from this video by Ivan Cappiello (one of past maintainers of Rigify)

https://www.youtube.com/watch?v=txJ-2zUQSTg

And of course, there is the Rigify manual which can be very useful. However it is has been lagging behind the actual development of the addon.
The 2.8 manual actually does not include images as of this writing.

https://docs.blender.org/manual/en/dev/addons/rigging/rigify/index.html?highlight=rigify

So it may be better to look at the old version of the manual
https://archive.blender.org/wiki/index.php/Extensions:2.6/Py/Scripts/Rigging/Rigify/#Advanced_Usage